Reproducing or recording high-density information by utilizing a laser beam has been mainly put into practical use as an optical disk.
The optical disks can be broadly classified into a reproduction-only type, a recordable type and a rewritable type. The reproduction-only type is put into practical use as a compact disk and a laser disk, and the recordable type and rewritable type are put into practical use as a document file, a data file and so on. The rewritable optical disks are mainly classified into a magnet-optical type and a phase-change type. The phase-change optical disk exploits a reversible state change occurring to a recording layer between an amorphous state and a crystalline state (or between crystals and crystals of further different structure) due to irradiation of the laser beam. To be more specific, the irradiation of the laser beam changes at least one of a refractive index and an extinction coefficient of a thin film so as to perform recording. And amplitude of transmitted light or reflected light changes in this portion so that transmitted light volume or reflected light volume leading to a detection system consequently changes, which is detected so as to reproduce a signal.
In general, a state in which a recording layer material is in the crystalline state is an unrecorded state, where the laser beam is irradiated to put the recording layer material in the amorphous state by melting and rapidly cooling it so as to record the signal. In the case of erasing the signal, laser beam power lower than that for recording is irradiated to put the recording layer in the crystalline state. In general, a chalcogen compound is often used as the recording layer material. The recording layer made of the chalcogen compound is formed as a film in an amorphous state, and so it is necessary to put the entire recording area in an unrecorded state by crystallization in advance. This entire crystallization is called initialization. An initialization process is incorporated into a part of a disk manufacturing process, and the recording layer is put in the crystalline state by using the laser beam or a flash light source. In the case of using the laser beam, the laser beam is irradiated while rotating the disk, and is focused on an information layer. The optical head position is radially deviated on the disk so that the entire disk surface is initialized.
There are proposals of a one-side two-layer configuration and a manufacturing method thereof from a viewpoint of increasing a recording capacity of the optical disk (Japanese Patent Application No. 10-132982, Japanese Patent Application No. 2000-400442 (unpublished) and Japanese Patent Application No. 2002-260307 for instance. The disclosure of Application No. 2000-400442 is included in Application No. 2002-260307). Japanese Patent Application No. 2000-400442 describes the manufacturing method of the phase-change optical disk of the one-side two-layer configuration using blue-violet laser light.
The entire disclosure of Japanese Patent Application No. 10-132982 and Japanese Patent Application No. 2002-260307 are incorporated herein by reference in its entirety.
It is a disk characteristic required of a one-side multilayered recording medium that, through the information layer on an incident side of the laser beam of recording and reproducing the information, it must record and reproduce the information in the information layer which is at the back thereof. For that reason, the layer on the front side especially requires a high transmittance (50 percent for instance), and so the recording layer in the information layer must inevitably be thin for that sake.